A microphone unit of a condenser microphone has a very high impedance and thus contains an impedance converter such as an FET (Field Effect Transistor) or a vacuum tube. Normally, a phantom power source is used for the condenser microphone. A microphone sound signal is output via a balanced shield cable for the phantom power source.
To connect to the balanced shield cable, a microphone case (in a hand-held microphone, a microphone grip) is provided with a 3-pin type output connector (see, for example, Japanese Patent Application Publication No. H11-341583). The output connector is defined in EIAJ RC-5236 ″ latch lock type round connector for audio equipment. The configuration of the output connector will be described with reference to FIGS. 3 to 5.
FIG. 3 is a sectional view showing an output connector installed in a microphone case. FIG. 4 is a front view showing the output connector removed from the microphone case. FIG. 3 is a section view of the output connector taken along line 3—3 in FIG. 4. FIG. 5 is a plan view of the output connector.
In this microphone, an output connector 10 comprises a disk-like connector base 11 consisting of an electric insulator such as a PBT (poly butadiene terephthalate) resin. The connector base 11 has three pins, that is, a grounding first pin E, a second pin SH for a hot side of a signal, and a third pin SC for a cold side of the signal, penetratingly installed by, for example, press fitting.
For a hand-held microphone, as shown in FIG. 3, the output connector 10 is installed in a connector housing cylinder 20 screwed to an end of a microphone grip. Normally, the microphone grip, including the connector housing cylinder 20, consists of a metal material such as brass. The microphone grip also serves as a shield case for contained electric parts.
An external thread 12 is formed in the connector base 11; the external thread 12 is used to fix the output connector 10 to the connector housing cylinder 20 and to electrically connect the grounding first pin E to the connector housing cylinder 20.
The external thread 12 is housed in a thread housing hole 13 drilled in the connector base 11 in its radial direction. The connector base 11 is provided with an earth terminal block 14 having an internal thread 14a engaged with the external thread 12 in the thread housing hole 13.
As shown in the plan view in FIG. 5, the earth terminal block 14 and the grounding first pin E are electrically connected together via a metal connection member 15. As shown in FIG. 3, the external thread 12 is rotated through a hole 21 drilled in the connector housing cylinder 20, using a driver (not shown). The male thread 12 is thus abutted against the periphery of the hole 21.
This electrically connects the grounding first pin E to the connector housing cylinder 20 via the external thread 12, earth terminal block 14, and metal connection member 15. Alternatively, as shown in FIGS. 4 and 5, a leaf spring 16 may be connected to the grounding first pin E to electrically connect the grounding first pin E to the connector housing cylinder 20; the leaf spring 16 contacts an inner surface of the connector housing cylinder 20.
When a microphone cable (balanced shield cable) drawn from a phantom power source (not shown) is connected to the output connector 10, if an intense electromagnetic wave is applied to the microphone or microphone cable, it may enter the microphone through the output connector 10. Then, the electromagnetic wave may be demodulated by an impedance converter and then output by the microphone as noise of an audible frequency.
The operation described below is conventionally used as a method for preventing an electromagnetic wave from entering the microphone through the output connector 10. A condenser is connected between the grounding first pin E and the hot-side second pin SH and between the grounding first pin E and the cold-side third pin SC; the condensers operate to short high frequencies. Further, the hot-side second pin SH and the cold-side third pin SC are connected to the microphone case such as the connector housing cylinder 20 via an inductor that inhibits the entry of high frequencies.
The above conventional technique can inhibit the entry of normal broadcasting electric waves, for example, electromagnetic waves of HF, VHF, or UHF without any problems. However, the recent prevalence of cellular phones and the like has increased the opportunities to use an electromagnetic wave of a higher frequency near the microphone.
The three pins E, SH, and SC of the output connector 10 are located directly inside the microphone case. Consequently, a high-frequency current may enter the microphone case through these pins, the high-frequency current resulting from an intense electromagnetic wave radiated by a cellular phone or the like. The high-frequency current then reaches an electronic circuit including an impedance circuit, by radiation or propagation to generate noise.
In particular, noise is made when a high-frequency current coming from the hot-side second pin SH or cold-side third pin SC is electrostatically coupled to the electronic circuit. In other cases, noise is made when high-frequency magnetic fields generated by a high frequency current flowing through the microphone case are magnetically coupled.